Transformers are often used as part of antenna structures. FIG. 1A is a diagram illustrating an example of a conventional antenna sub-system 100 including a transformer 110 connected to an antenna 120.
A common type of transformer used in antenna structures is the BALUN transformer. An example of a BALUN transformer 110 connected to the antenna 120 is shown in FIG. 1B. The transformer includes two electrically separate windings 112, 114 that generally are made of wire coils wound around the transformer's core. The transformer 110 converts between a balanced signal (two signals received at the electrical contacts 116a and 116b and referenced against one another) on the winding 114 and an unbalanced signal (one signal referenced against a ground 130) on the other winding 112. The goal of the BALUN transformer is to balance different elements of the antenna 120 relative to ground 130 to suppress reception of common mode interference, and to improve antenna directionality and similar properties. Unfortunately, however, the transformer itself can become a source of unwanted interference, particularly in precisely balanced antennas. Stray radiation (denoted by vector B in FIGS. 1A and 1B) may induce electromagnetic interference in the transformer windings 112, and/or 114 due to asymmetry of the coil layout and field gradients. The phenomenon is present even in transformers with toroidal cores, which are inherently self-shielding.
Conventional approaches to addressing this problem have centered on producing transformers with highly symmetric windings, using fully enclosed cores (e.g., a mushroom core), and improved shielding.